A circuit on a printed circuit board is created by placing a sheet of film having the circuit design over a copper-clad panel that is coated with photosensitive material. When the film is exposed to light, the photosensitive coating on the panel is chemically altered. After further processing, select portions of the copper on the circuit panel are etched away in accordance with the circuit pattern on the film. This creates a circuit pattern on the panel.
Printed circuit boards often include many layers of circuitry, and each separate circuit requires a different film. It is important that the all the circuits on the printed circuit board be properly aligned. Thus, when the printed circuit board is being created, the films having the circuit designs must be properly aligned. The films are usually aligned two at a time, one film being above the other (though not necessarily touching).
In addition, a film and a substrate may also have to be aligned. Examples of a substrate include a panel used to make a printed circuit board, a metal sheet for chemical milling, or flexible circuit bearing material, such as polyimide. One reason why a substrate and a film may have to be aligned is because the substrate may include holes or layers of circuitry, and the film must be aligned with these features on the substrate.
A film may also need to be aligned with a fixed reference plate, such as a punch press base. A film is aligned to a punch press base for the purpose of punching registration holes.
Registration schemes that align the films to each other or a film to a substrate are known, and one such scheme uses a camera. In the camera scheme, one film or substrate has a black spot on it, and the other film or substrate has a clear spot on it. The clear spot is larger in size than the black spot. The camera, which is placed above or below the film/film layer or film/substrate layer in vertical alignment with the clear spot, views the film or substrate having the black spot while the that film or substrate is moved. The camera produces a video signal, which is sent to a video capture board. The capture board conditions the video signal for readability by a computer, which is attached to the capture board. The computer determines when the clear spot is centered above the black spot, and, when this happens the two films or the film and substrate are properly aligned.
The camera scheme has several disadvantages. First, it is expensive because it requires a camera, lens, illumination, a precise camera mount, a video capture board, a powerful computer, and expensive software. Additionally, the camera scheme also requires a skilled operator because the camera must be properly focused and the light intensity must be properly set. The camera scheme is also slow, as it can take up to several seconds to align the films. Therefore, it is desirable to have a faster, lower cost system that requires minimal operator intervention to align films or substrates.